The present invention provides a laser diode within a multiple quantum well structure.
A conventional laser diode will be described with reference to FIGS. 1 and 2A through 2C. The conventional laser diode is provided as illustrated in FIGS. 1 and 2A. The conventional laser diode is formed on an n-type InP substrate 1. A surface of the n-type InP substrate 1 is formed with a grating 9. An optical guide layer 2 is provided which extends over the grating 9. The optical guide layer 2 is made of InGaAs which has a wavelength composition of 1.2 micrometers. An active layer is provided, which extends over the optical guide layer 2. The active layer has a multiple quantum well structure. An optical guide layer 4 is provided, which extends over the active layer 3. A p-type InP cladding layer 5 is provided, which extends over the optical guide layer 4. A p-type InP layer 6 is provided which extends over the p-type InP cladding layer 5. A p-type InGaAsP contact layer 7 is provided which extends over the p-type InP layer 6. A p-type electrode is provided on the p-type InGaAsP contact layer 7.
The following descriptions will focus on the multiple quantum well structure of the active layer 3 with reference to FIGS. 2B and 2C. The multiple quantum well structure comprises alternating laminations of electroluminescence well layers 31 and potential barrier layers 32.
The electro-luminescence (also spelled "electroluminescence" below ) well layer 31 comprises a +0.8%-strained InGaAs layer which has a thickness of 5.5 nanometers and a wavelength composition of 1.72 micrometers. The potential barrier layer 32 comprises a non-strained InGaAs which has a thickness of 4.0 micrometers and a wavelength composition of 1.15 micrometers. The electro-luminescence well layer 31 has an energy band gap E.sub.g1 of 0.80 eV, where energy band gap is defined as a difference between a ground level of electrons in conduction band and a ground level of holes in valence band. The energy band gap E.sub.g1 of 0.80 eV of the electro-luminescence well layer 31 is converted into a wavelength of 1.55 micrometers.
The above electro-luminescence well layer 35 having the small energy band gap E.sub.g1 of 0.80 eV shows electro-luminescence at a wavelength of 1.55 micrometers. This electro-luminescence is caused by the induced transition by electron-hole recombination. This means that the electro-luminescence well layers become necessarily deficient in carriers. This makes the electro-luminescence well layers be deficient in carriers for the electro-luminescence. This causes a considerable variation in refractive index of a laser medium. This enlarges an active wavelength chirping.